In many industries employing refrigeration systems, such as the airline, trucking, shipping and building industries, conventional refrigeration technology is based on the vapor-compression cycle. In aircraft, for example, a vapor-compression cycle air chiller is typically mounted either on top of a galley of the aircraft, such as in the crown area, or below the cabin floor, such as in the cargo area between floor beams. To cool consumables such as food and beverages, the air chiller is typically connected to one or more galley food storage compartments via a series of air supply/return ducts, which collectively form a closed-loop system. In operation, the air chiller is essentially a unitized air conditioner similar in principle to the conventional window-unit air conditioners typically mounted in a window of a house. The objective is to maintain the temperature of the consumables between 0° C. and 7° C., or between 0° C. and 5° C. (or 4° C. in many European countries) as may be required in the future.
In order to maintain the consumables at a temperature within the proper temperature range, a desired temperature difference must exist between the warmer aircraft cabin atmosphere and the cooler galley food storage compartments atmosphere. This temperature difference causes heat energy in the warmer aircraft cabin to flow into the cooler galley food storage compartments via a combination of heat transfer mechanisms. Conventionally, the rate of this heat transfer (or heat load) at any given temperature differential is governed by the effective net insulation between the warm and the cool atmospheres. In this regard, the vapor-compression cycle air chiller typically must be able to remove this heat load from the cooler food storage compartments in order to maintain the desired temperature differential, thereby keeping the consumables at a temperature within the proper temperature range. The heat removed by the air chiller is rejected to the atmosphere in either the airplane cargo compartment or the cabin crown, depending on the location of the air chiller.
Conventionally, the vapor-compression cycle air chiller is an air-to-air system. In this regard, a fan in the air chiller unit circulates air from the galley food storage compartments via the air return ducts through an evaporator coil mounted inside the air chiller. Inside the evaporator coil, cold coolant, such as cold R134a refrigerant (gas phase), soaks up the heat from the air flowing across the evaporator coil. As the air flows across the evaporator coil, the air loses heat energy to the coolant. The cold air is then circulated back into the galley food storage compartments via the air supply ducts. Once inside the galley food storage compartments, the cold air soaks up the heat energy inside the food storage compartments. The process can then be repeated in a continuous manner in order to maintain the desired temperature differential.
As will be appreciated, once the coolant receives the heat energy from the air flowing across the evaporator coil, the heat energy must be rejected from the coolant. In this regard, the gaseous coolant becomes superheated as it soaks up the heat energy through the evaporator coil. The superheated gaseous coolant is then typically drawn into a compressor within the air chiller. The compressor then does work on the gaseous coolant by forcing the gaseous coolant into a smaller volume by applying external pressure. As a result, the temperature and pressure of the gaseous coolant is greatly increased. The high temperature and pressure gaseous refrigerant is then circulated through a condenser located in the air chiller unit. As the gaseous refrigerant flows through the condenser coil, a fan blows ambient air across the condenser coil to cool the hot, gaseous refrigerant. As the refrigerant circulates through the condenser coil, it loses heat energy to the ambient air such that the refrigerant changes state from a high-pressure, super-heated gas to a saturated high pressure liquid as it leaves the condenser coil and enters a liquid receiver. The liquid refrigerant travels through the high-pressure liquid line to an expansion valve (or in some systems, a capillary tube) and is expanded into a saturated gas before it re-enters the evaporator coil.
Whereas refrigeration systems employing vapor-compression cycle air chillers are adequate for maintaining consumables at a temperature within the proper temperature range, such refrigeration systems have drawbacks. In this regard, the heart of the vapor-compression cycle air chiller is the compressor. Operation of the compressor as well as the fan blowing air across the condenser, however, undesirably consumes significant amounts of electrical energy. Also, the compressor is typically a complicated mechanical device, which is noisy and prone to failure. In addition, operation of the air-chiller rejects heat into the cabin environment, which can be problematic for the environmental control system (ECS) during ground operations. In this regard, ECS packs that provide cooling to the airplane cabin and equipment during ground operation are typically located under the airplane wing box, which stores airplane fuel. As such, the harder the ECS system has to work in hot climates, the more heat the ECS system rejects into the airplane fuel. Rejecting heat into the airplane fuel may cause undesirable fuel vaporization in a partially full fuel tank which, in some instances, has been linked to incidents of airplanes exploding on the ground due to vaporized fuel being ignited by sparks from malfunction fuel pumps.